AC motors spin at a speed determined by the number of poles and the frequency of the applied AC current. The speed in revolutions per minute (RPM) is equal to 120× frequency (Hz) divided by the number of poles. For example, a motor with four poles operating at 60 Hz, would have a nominal speed of 1800 rpm. The operating speed of traditional AC motors is relatively constant, though in practice, the loaded speed does vary.
The rotational speed of DC motors, on the other hand, varies with supply voltage. By reversing the polarity of the supply voltage, a DC motor will reverse direction. Speed control, therefore, is a fairly simple matter with DC motors. When speed control is important, and the ability to reverse the direction of rotation is also needed, DC motors provide one effective option.
The oil industry is one area where high-power rotational motors with reliable speed control are used. An oil well is drilled by rotating a drill string with a drill bit at its end. Today, it is common for a variety of exploration tools to be mounting in the drill string, typically near the drill bit. These tools measure temperature, pressure, density of the formation, resistivity or conductivity of the formation, and various other parameters of interest to oilfield exploration engineers.
In an oilfield drilling operation, it is desirable to control the speed of the drill motor. This can be important for optimum effectiveness of the drilling bit, for removal of cuttings, and for the operation of tools installed in the drill string. Large DC motors traditionally have been used in the oilfield for this purpose. These motors are not very efficient, but they do provide reasonably good control of the rotational speed of the drill string. These motors also provide high torque, which is crucial in this industrial setting.
Variable frequency drive (VFD) AC motors have become increasingly popular in recent years, including in the oilfield industry. VFD motors are a good alternative to DC motors, in large part because the VFD motors are more efficient. Improvements in the technology in recent years have made large VFD motors a reliable, efficient option in many heavy industries. The oilfield industry has been opting for large VFD motors more and more in recent years.
To supply VFD motors, two conversions are done. First, the AC supply is converted to DC, and then the DC is converted to a variable frequency AC signal. In the most common arrangement, the variable frequency AC signal is made up of a series of DC pulses. Pulse width modulation of a DC output is used to create a simulated AC sine wave signal. The DC polarity is reversed to create the negative portion of the simulated sine wave.
This process involves a great deal of high speed switching. In high-power applications, the switching components may have to switch on and off thousands of times per second, and may rise and fall by hundreds of volts with each switch. This type of switching produces a great deal of harmonic and switching noise in the system. These noise components of the total signal will be carried by the conductors from the power supply to the motors.
The VFD noise can cause problems with electronic systems operated in the same physical area. Computer equipment can experience problems. Control and monitoring equipment also may experience problems due to the VFD noise. VFD motors offer important benefits, but the problems caused by the VFD noise must be controlled, or this problem may outweigh the benefits of a VFD system.
To limit the transmission of the noise signals, shielded power cables are typically used application were VFD noise poses a problem. Again, the oilfield industry provides a good example. During the oil drilling operation, computers and other electronic equipment are used to monitor and evaluate various parameters. VFD noise can cause serious problems in the oil drilling situation if it is not controlled. Shielded power cables are often used for this reason in oilfield applications where VFD motors are used.
A typical shielded cable application in the oilfield might involve use of single, shielded power cables running from the VFD power supply to the VFD motor. The cables are hard-wired at each end, so no separate inline connectors are used. The shielding is grounded at one or both ends of the run. The internal, shielded, power conductor supplies the VFD current to the VFD motor. The continuous run of shielding on the power cable contains most of the potentially harmful VFD noise.
This typical arrangement will not work, however, if a connection is needed somewhere between the supply and the drive motor, or at either end of the power cable. For example, if the run from the VFD power supply to the VFD motor is too long for a single cable, it is necessary to use some type of inline connector to piece together the different sections of shielded cable. This may be a fairly common situation because the shielded cable used in oilfield and other heavy industries tends to be quite large and heavy. Such cable may weigh several pounds per foot, making long cable runs quite heavy and unwieldy. Using shorter sections of cable connected together with separate connectors is one way of addressing this problem.
Cable connections also may be needed at the VFD motor or at the supply. Use of a connector at these points allows for easier replacement of a cable, when compared to a hard-wired arrangement. In oilfield drilling operations, the drive motor may be moved up and down during the drilling process. The drive motor may also need to be moved to another position for service or inspection. With so much movement, the connections between the cable and the drive motor will be subject to stress and may fail after extended use. In addition, if the drive motor is to be moved for inspection or service, there may be a need to disconnect the drive motor from its supply cables. These connection and disconnection operations are much easier to accomplish is a separate connector is used, as opposed to hard-wiring the supply cables to the drive motor.
If a nonshielded connector is used, some of the noise found in the VFD power lines will be transmitted to various items that may be damaged by such noise. Computers and other electronic equipment may be vulnerable to such damage. It is, therefore, highly desirable to ensure than the entire electrical path from the VFD power supply to the VFD motor, including all connections, is fully shielded. Shielded power cables are relatively easy to find, but there remains a need for high-power shielded connectors.
The need for an inline or end-of-cable connector in high-power VFD applications poses a problem. Low power shielded cable connectors are well known. Such connectors are used widely on home cable television and Internet systems. The small, shielded connectors used in such applications provide a continuous shield for any noise that exists on the cable signal.
In a typical low power shielded connector, the cable has a small internal core conductor that carried the signal of interest. An insulator surrounds the core conductor, and a braided shield surrounds the core insulator. Another insulator, typically the outer covering of the cable, is positioned over the braided shield wires. The shielded connector connects the braided shield wires to the outer shell of the connector and connects the core conductors while providing an insulation layer between the core conductors and the shell of the connector. In this manner, a continuous electrical path is provided for both the core conductor and the braided shield, with these two paths being electrically insulated from each other.
The same concept is possible, and needed, in the high-power VFD motor context. It is, however, a huge step to go from the small, shielded connectors used with home cable television systems to the sort of shielded connector needed for a high-power VFD situation. The core conductor in a home television cable is not much larger than a piece of thread or fishing line. The cable is light, the shielding is very thin and easily handled. The current capacity of these systems, and the connectors used with these systems, is quite low.
In an oilfield VFD application, on the other hand, the cables can weigh hundreds of pounds. The core power conductors can be an inch thick or more and are very stiff. The shielding used in these high-power applications is much heavier and harder to work with than the thin shielding braid found on a home television cable. Cutting, crimping, and other typical tasks associated with making up electrical connectors all take on a very different nature when large, high-power cables are involved.
One particular challenge found in the high-power VFD application that is not present with low power cable television connectors is the difficulty in making up nearly identical connections repeatedly. Given the size, weight, and stiffness of the large power cables involved in high-power VFD applications, it is not practical to use a connector that requires precise and consistent positioning of all the connections between the connector and the supply cable. It is, therefore, highly desirable for a high-power VFD connector to allow for some variance in the positioning of the connections involved, while still providing a reliable, fully shielded connector.
Because the supply cable used in high-power VFD applications is so heavy and stiff, it is almost impossible to make up a connection with such cable if a quick turn or change of direction is required. Consider, for example, a connection made in a physical space where the supply cable must turn 45° immediately after the point of connection. It may not be possible to bend the cable to create this sharp a turn. There is a need, therefore, for a connector that solves this problem by allowing for use of heavy, shielded power cables, while providing the ability to make sharp bends or turns.
Finally, it is desirable for this connector to have an internal insulator between the shielded shell of the connector and the internal power conductor. Such an insulator should allow for access to lug bolts while also providing the capability to fully isolate, electrically, the internal power conductor once the connection has been made up. The insulator should be reliable and easy to use.
The present invention provides the high-power shielded connector needed for use with high-power VFD motors and power supplies. In a preferred embodiment, the connector includes a high-power, single-pole electrical connector; an electrically conductive, generally cylindrical outer shell having an internal electrical contact region; an electrically insulating layer positioned between the single-pole connector and the electrically conductive outer shell; and, a generally cylindrical shielding trap configured to provide an electrical connection between a shielding material of a high-power, electrical cable and the internal electrical contact region of the electrically conductive outer shell.
The method of connecting a high-power, shielded electrical cable to the connector includes stripping the supply cable to expose its layers as follows: approximately 1.5 to 1.75 inches of a core conductor; approximately 0.75 to 1.25 inches of a core conductor insulation; and, approximately 0.25 to 0.75 inches of a shielding layer. A high-power, single-pole electrical connector is connected to the exposed portion of the core conductor. A shielding trap is connected to the exposed portion of the shielding layer, such that the core conductor insulation is positioned between the shielding trap and the high-power, single-pole electrical connector. An insulating barrier is positioned around at least a portion of the high-power, single-pole electrical connector; and, an electrically conductive outer shell is positioned over the insulating barrier, the high-power, single-pole electrical connector, and the shielding trap such that the shielding trap is in electrical contact with the outer shell and the outer shell is electrically isolated from the high-power, single-pole electrical connector.